Heat exchanges such as condensers, evaporators and the like for use in car coolers, air conditioning systems etc. usually comprise a number of heat exchange tubes arranged in parallel between two headers, each tube joined at either end to one of the headers. Corrugated fins are disposed in an airflow clearance between adjacent heat exchange tubes and are brazed to the respective tubes. The heat exchanger is typically made of aluminium or an aluminium alloy.
In the past, flat refrigerant tubes have been manufactured by folding a brazing sheet clad on the outside with a brazing material layer. The refrigerant tubes, the headers and the fins, were then assembled and heated to the brazing temperature at which the clad layer melts and joins together the fins, refrigerant tubes and headers into a brazed assembly.
It is envisaged gases such as carbon dioxide will be used as cooling medium in air-conditioning systems. The use of carbon dioxide will lead to an increase in operating temperature and pressure of the air-conditioning units. The above described conventional brazed tubes might not withstand under all circumstances the encountered operating pressures and temperatures. For the existing carbon dioxide based prototypes, the heat exchange tubes have therefore been made of a hollow extrusion comprising flat upper and lower walls and a number of reinforcing walls connecting the upper and lower walls. A disadvantage of the extrusion technique is that the walls cannot be made as thin as desired. Further, an extruded tube cannot be clad with brazing material, so the corrugated fins must be clad in order to allow brazing to the heat exchange tubes, which is expensive due to the large surface area of the fins. In addition, a tube made of brazed sheet or plate is stronger and more resistant against corrosion than extruded tubes.
U.S. Pat. No. 5,931,226 discloses a refrigerant tube or fluid tube for use in heat exchangers comprising a flat tube having upper and lower walls and a plurality of longitudinal reinforcing walls connected between the upper and lower walls. The reinforcing walls consist of ridges projecting inward from the upper or lower wall and are joined to the flat inner surface of the other wall. The ridges are produced by rolling an aluminium sheet clad with a brazing filler metal layer over at least one of its opposite surfaces with a roll having parallel annular grooves. Parallel refrigerant or fluid passages are defined between adjacent reinforcing walls. Further, the reinforcing walls include a plurality of communication holes for causing the parallel refrigerant passages to communicate with one another. In another embodiment, each reinforcing wall is formed by a ridge projecting from the upper wall and a ridge protecting from the lower wall, joined to each other at their respective top ends. The upper and lower walls are either produced separately or in one sheet, whereby the flat refrigerant tube is manufactured by folding the sheet longitudinally at its midpoint like a hairpin.
U.S. Pat. No. 5,947,365 describes a process for producing a similar flat heat exchange tube having a plurality of reinforcing walls formed of ridges projecting from the lower wall. The upper and lower walls are connected by brazing the tops of the ridges on the lower wall to the upper wall. In order to strengthen the brazed connection between the reinforcing walls and the lower surface of the upper wall and to prevent the creation of a clearance space therebetween, the lower surface of the upper wall is provided with smaller longitudinal ridges with which the upper surfaces of the reinforcing walls come into contact to eliminate the clearances and thereby to insure the existence of a continuous brazed connection between each reinforcing wall and lower surface of the upper wall.
A different method of producing reinforcing walls in a flat refrigerant tube for use in heat exchangers is shown in U.S. Pat. No. 5,186,250. The tube comprises one or more curved lugs integral with and protruding inwardly from an inner surface of each plane wall, and the curved lugs respectively have innermost tops so that the innermost tops protruding from one plane wall bear against the inner surface of the other plane wall or against the tops of the other curved lugs protruding from the opposite plane wall. The purpose of such protruding lugs is said to improve the pressure resistance of the tube while minimizing its height and thickness.
In the production of these known tubes, it is difficult to achieve a precise alignment between the ridges on the upper and lower walls, especially in those embodiments where two ridges protruding from opposing walls have to be joined head-on. Further, the brazed connection between the ridges or between the top of a ridge and the lower surface of the opposing wall is not very strong.